Fix namespace typo 'PHPunit' to 'PHPUnit'

This path of `ResultPrinter.php` is src/NyanCat/PHP<strong>Unit</strong>/ResultPrinter.php

but namespace and config specification is PHP<strong>unit</strong>

Hope to fix along [PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md)